As this kind of diaphragm of the electrostatic capacity type pressure gauge, in order to achieve a pressure measurement having a high precision, there has been generally employed a thin film having a uniform thickness equal to or less than 0.1 mm as a whole in a range between 25 and 125 μm. Further, a diameter thereof is very small such as about 1 inch (25.4 mm).
In order to attach the diaphragm made of the thin film and having the small diameter to the tubular case in a fixed state, in general, there has been conventionally employed a means for directly firmly attaching a thin outer peripheral edge portion of the diaphragm to an end surface portion around the opening on the one end of the tubular case in accordance with a Tig welding or the like.
However, in a case of employing the general means for welding the thin outer peripheral edge portion to the end surface portion of the tubular case so as to be firmly attached as mentioned above, the tubular case is hard to be molten and the diaphragm is easily molten due to a difference between a groove volume of the tubular case and a groove volume of the outer peripheral edge portion of the diaphragm which are to be welded and firmly attached to each other. Accordingly, a weld penetration of the welded position becomes uneven and a poor weld tends to be generated, so that a deterioration of a production yield ratio tends to be caused. Further, in the diaphragm having the small thickness, the outer peripheral edge portion is thermally molten rapidly and a weld heat makes progress toward a center portion of the diaphragm. However, since a progressing degree of the weld heat is not uniform in all the circumference of the diaphragm but is dispersed in a circumferential direction, a heat strain such as an annular crimp which does not have a shape similar to the outer peripheral edge portion is generated in an inner periphery of the weld outer peripheral edge portion, and a distortion displacing amount of the diaphragm is adversely affected by the heat strain. As a result, there is caused a problem that it is impossible to secure a predetermined pressure measuring precision.
The poor weld and the generation of the heat strain can be reduced to a certain level by an improvement of a welding skill, however, a deterioration of the yield ratio and an increase of a production cost are caused in the light of a production technique, and an accurate measuring precision cannot be secured at all.
Accordingly, there has been conventionally proposed a diaphragm in which radial or multiple concentric waves or crimps are previously and positively formed in a whole region of a center portion thereof by press molding the center portion except an outer peripheral edge portion, as a diaphragm of an electrostatic capacity type pressure gauge (for example, refer to Patent Document 1).
Patent Document 1: U.S. Pat. No. 4,434,203